Cleanups to Kconfig definitions for KVM

* replace HAVE_KVM with an architecture-dependent symbol, when CONFIG_KVM
  may or may not be available depending on CPU capabilities (MIPS)

* replace HAVE_KVM with IS_ENABLED(CONFIG_KVM) for host-side code that is
  not part of the KVM module, so that it is completely compiled out

* factor common "select" statements in common code instead of requiring
  each architecture to specify it